Discharge lamps are efficient light sources; such lamps, when used as light sources for film and television recording, as well as for theatrical and similar uses, usually have power ratings between about 100 W and several thousand W. These lamps may be single-based or double-based. The discharge vessel itself, which surrounds a discharge chamber, can be surrounded by an outer bulb. The wall loading of the discharge vessel is typically between about 50-100 W/cm.sup.2 at a temperature in the order of about 1000.degree. C. If an outer bulb is used, the loading on the outer bulb is, typically, half as large.
The light spectrum emitted by these lamps closely approximates the characteristics of daylight. The color temperature is between about 5000-6000K, with excellent color rendition; the color rendition index Ra is usually above 90. The emitted spectrum has a relatively high proportion of continuous radiation with dense spectral lines derived from rare-earth atoms in the fill of the discharge chamber, which are superimposed above the remainder of the spectrum.
Lamps of this type are described in the referenced patents and application assigned to the assignee of the present invention, the disclosures of which are hereby incorporated by reference, namely U.S. Pat. No. 5,164,630, Greiler et al, and U.S. Ser. No. 07/805,858, filed Dec. 10, 1991, Genz, a co-inventor of the present application, issued as U.S. Pat. No. 5,323,085.
The lamps have excellent radiation and operating characteristics. They do, however, have a problem in that about 10% of the radiation energy is within a spectral band which is undesired. Short-wave radiation, and especially very short wave UV-C radiation, is particularly dangerous for human skin. In natural sunlight, the amount of UV radiation within the C band is small. Another problem is the UV radiation within the UV-B band, which may lead to sunburn. The proportion of UV radiation in the UV-A band can be tolerated, if it is not too high. UV-C radiation has a wave length of less than 280 nm; UV-B radiation has a wave length between 280 mn and 315 nm; and UV-A radiation has a wave length of between 315 nm and 380 nm. Normal quartz glass is transparent to the UV radiation in the C, B and A bands.
It has been proposed to reduce the UV portion of radiation by coating the bulb or vessel of a discharge lamp with metal oxides, particularly titanium oxide or zinc oxide--see U.S. Pat. No. 5,051,650, Taya et al. It has also been proposed to use titanium and/or cerium oxide as a doping of a quartz-glass bulb, by adding about 10-300 ppm, see European Patent EP-A 389 717, Saito et al.
U.S. Pat. No. 4,336,048, Van deer Steen et al, to which European Patent Disclosure Document 0 019 327 B1 corresponds, describes a process of making doped glass, in which a melt of quartz powder and doping material is prepared which is then sintered to provide a concentrate. The doped glass is made by melting the quartz powder together with the sintered concentrate in a protective gas atmosphere formed of helium and hydrogen (He-H.sub.2) which, for example, is described in the U.S. Pat. No. 3,764,286, Antczak et al. A tungsten boat is usually used as the melting furnace. This process has the disadvantage that the manufacture of the doped quartz glass requires two melting steps or melting processes, both of which are highly energy-intensive.